The invention relates to a biaxially oriented polyester film which has at least three layers and which, together with electromagnetic properties which are better than those of prior art films, has improved abrasion properties, and which is cost-effective to produce and is built up from at least one base layer B and, applied to both sides of this base layer, outer layers A and C, where these outer layers have Ra values and Rz values and a defined number of elevations of defined height, and the film has a haze per unit thickness of xe2x89xa60.4%/xcexcm. The invention also relates to a process for producing the film and to its use, in particular as magnetic tape film.
Due in particular to their excellent mechanical properties, polyester films have for a long time been used as substrate for magnetic recording materials. An ideal magnetic recording medium with good electromagnetic properties has a surface which is as smooth as possible. For good processing performance during coating and in subsequent use, and in order to achieve good abrasion performance, the surface should have a degree of roughness. Monofilms (single-layer films) can fulfil these requirements, which are per se contradictory, only if there is a degree of readiness to compromise, since in these films the optimization of one of the properties always disadvantages the other property.
Coextruded films having more than one layer (AB, ABA and ABAxe2x80x2) with which it is possible to achieve xe2x80x9cdual-surfacexe2x80x9d properties have now also been disclosed, however. By this means it is possible, to a limited extent, to attribute differing properties (roughness, topographies) to the two film surfaces, but the contradictory requirements for improved electromagnetic properties together with good abrasion performance are not met.
EP-A-0 135 451, for example, describes a xe2x80x9cdual-surfacexe2x80x9d film of A/B type where the two film surfaces have differing Ra values. However, these films have the disadvantage of not being cost-effective to produce, since the two layersxe2x80x94corresponding to two monofilms lying one upon the otherxe2x80x94have to be provided with particle systems and therefore do not permit any cost saving in comparison with monofilms. The quality of the magnetic tape film will be impaired, furthermore, by the regenerated film material (regrind) which arises in all commercial production processes and has (necessarily) to be reused, and which must be incorporated into at least one surface layer of the A/B film.
EP-A-0 609 060 and EP-A-0 663 286 achieve an improvement with respect to this quality problem by the principle of A/B/A coextrusion, enabling the effect of the regenerated material on the film surface to be reduced by the screening effect of the two outer layers A. However, A/B/A films outwardly exhibit the character of (thin) monofilms, i.e. the advantage of creating differing surface properties has been lost. However, a xe2x80x9cdual-surfacexe2x80x9d character can be introduced to a limited extent in A/B/A films via differing thicknesses of the outer layers A. Films of this type are then termed ANB/Axe2x80x2 films (xe2x89xa0A/B/C). A disadvantage of these films, however, is that flexibility for the different design of the surface topographies of the two film surfaces remains very restricted. There is also no effect on properties such as electromagnetic properties or abrasion resistance, as is shown in Comparative Examples 1 to 6.
A few A/B/C films and A/B/C/B films have also been produced and described (EP-A-0 502 745), but here again at least one surface layer comprises regenerated materialxe2x80x94which has the disadvantages described abovexe2x80x94or else the information is unspecific for A/B/C films (EP-A-0 347 646).
It was therefore an object of the present invention to provide a coextruded, biaxially oriented polyester film which has more than one layer and which is suitable as substrate for magnetic recording media and, at the same time, has a smoother surface (for good electromagnetic properties of the magnetic tape) and a rougher surface (for good processing performance on high-speed coating systems and good running performance subsequently when the tape is in operation) and low abrasion. It should moreover be possible to produce the high-quality film (with low drop-out values) cost-effectively (at low cost).